If you want to do three, Psychology, German and Geography are good options. Psych is obviously a 'soft' subject, but you have two 'hard' subjects so it's fine. Languages are always good, and the combination of it altogether makes a lot of sense.
If you want to do 4 you could do media studies? To be honest, it's really really looked down upon, and imo there's no point unless you genuinely want to do it.
I'd say to either do an extra AS or an EPQ. Easier than 4 subjects at A2, but still looks impressive. You can always do AS Maths - the mechanics in AS isn't too bad to be honest, but at A2 it's harder and there's much more mechanics than stats.